Resonance-Raman and lattice-dynamics studies of single-crystal PdO

Photocatalytic activity enhancement of two-step and one-pot synthesis of Pd/ZnO nanocomposites: an experimental and DFT study

Pd/ZnO nanocomposites were successfully synthesized by means of one and two pot synthesis and applied in the photodegradation of Rh6G. The nanocomposites were characterized by XRD, SEM, TEM, FTIR and micro-Raman spectroscopies. It was found the presence of PdZn2, PdO and agglomerated particles in the support surface for the Palladium-based nanocomposites fabricated by one-pot route; the two-step method allowed the formation of spherical Pd nanoparticles, with homogeneous distribution in the nanocomposite matrix, with an average size of 2.16 nm. The results show higher photocatalytic efficiency for the samples fabricated under the two-step approach compared to the one-pot synthesis. Based on experimental results, density functional theory (DFT) calculations were carried out to understand the enhancement photocatalytic of Pd/ZnO nanocomposites. To achieve it, the ZnO (001) and (101) surfaces were built and decorated by different Pd coverages. The theoretical results indicated two different photocatalytic mechanisms. In ZnO (001) case, the electrons flowed from surface to Pd, generating the superoxide radical anion (⋅O2-). Furthermore, the density of states of the ZnO (001) surface was modified by impurity Pd-d states at proximity to the conduction states, which may work as electron acceptors states. On the other hand, we found that the electrons flow from Pd to ZnO (101) surface, inducing the formation of ⋅OH and ⋅O2- for the degradation of Rh6G. The density of states of the ZnO (101) revealed a reduction in its bandgap, due to Pd-d states localized above valence states. Hence, our theoretical results suggest that the Pd-d states may facilitate the mobility of electrons and holes in (001) and (101) surfaces, respectively, reducing the rate of charge recombination.

Unlocking High-Efficiency Methane Oxidation with Bimetallic Pd-Ce Catalysts under Zeolite Confinement

Catalytic complete oxidation is an efficient approach to reducing methane emissions, a significant contributor to global warming. This approach requires active catalysts that are highly resistant to sintering and water vapor. In this work, we demonstrate that Pd nanoparticles confined within silicalite-1 zeolites (Pd@S-1), fabricated using a facile in situ encapsulation strategy, are highly active and stable in catalyzing methane oxidation and are superior to those supported on the S-1 surface due to a confinement effect. The activity of the confined Pd catalysts was further improved by co-confining a suitable amount of Ce within the S-1 zeolite (PdCe_0.4@S-1), which is attributed to confinement-reinforced Pd-Ce interactions that promote the formation of oxygen vacancies and highly reactive oxygen species. Furthermore, the introduction of Ce improves the hydrophobicity of the S-1 zeolite and, by forming Pd-Ce mixed oxides, inhibits the transformation of the active PdO phase to inactive Pd(OH)₂ species. Overall, the bimetallic PdCe_0.4@S-1 catalyst delivers exceptional outstanding activity and durability in complete methane oxidation, even in the presence of water vapor. This study may provide new prospects for the rational design of high-performance and durable Pd catalysts for complete methane oxidation.

Palladium/Carbon Nanofibers by Combining Atomic Layer Deposition and Electrospinning for Organic Pollutant Degradation

As organic dyes are a major source of pollution, it is important to develop novel and efficient heterogeneous catalysts with high activity for their degradation. In this work, two innovative techniques, atomic layer deposition and electrospinning, were used to prepare palladium nanoparticles (Pd NPs) supported on carbon nanofibers (CNFs). The sample morphology was investigated using scanning and transmission electron microscopy. This showed the presence of nanofibers of several micrometers in length and with a mean diameter of 200 nm. Moreover, the size of the highly dispersed Pd NPs was about 7 nm. X-ray photoelectron spectroscopy visually validated the inclusion of metallic Pd. The prepared nano-catalysts were then used to reduce methyl orange (MO) in the presence of sodium borohydride (NaBH₄). The Freundlich isotherm model was the most suitable model to explain the adsorption equilibrium for MO onto the Pd/CNF catalysts. Using 5 mL MO dye-solution (0.0305 mM) and 1 mL NaBH₄ (0.026 mM), a 98.9% of catalytic activity was achieved in 240 min by 0.01 g of the prepared nano-catalysts Pd/C (0.016 M). Finally, no loss of catalytic activity was observed when such catalysts were used again. These results represent a promising avenue for the degradation of organic pollutants and for heterogeneous catalysis.

Catalytic Performance of Palladium Supported on Sheaf-Like Ceria in the Lean Methane Combustion

Sheaf-like CeO₂ (CeO₂-S) in microscale was prepared by the hydrothermal method, and then etched with KOH aiming to obtain an imperfect fluorite structure (CeO₂-SK) with high content of oxygen vacancies and oxygen mobility. With CeO₂-S and CeO₂-SK as supports respectively, a modified colloidal deposition method was employed to obtain Pd/CeO₂ catalysts for being used in lean methane combustion. According to the inductively coupled plasma (ICP), N₂ physisorption and scanning electron microscopy (SEM) results, the Pd supported catalysts are very similar in their Pd loading, surface area and morphologies. SEM and transmission electron microscopy (TEM) results revealed various nanorods exposed CeO₂ (110) and (100) facets on Pd/CeO₂-SK surface after KOH etching. Raman spectra and H₂-temperature programmed reduction (H₂-TPR) results indicated that Pd/CeO₂-SK catalyst has a much higher content of catalytic active PdO species than Pd/CeO₂-S catalyst. It was also found that the catalytic performance of Pd/CeO₂ in lean methane combustion depends greatly upon the exposing crystal planes and oxygen vacancies content of sheaf-like CeO₂, and Pd/CeO₂-SK exhibits higher activity than Pd/CeO₂-S. The larger amount of CeO₂ (110) and (100) planes on Pd/CeO₂-SK surface can enhance the formation of oxygen vacancies, active Pd species and migration of lattice oxygen, which all evidently improve the redox ability and catalytic activity of the Pd/CeO₂-SK catalysts in lean methane combustion.

Taming the stability of Pd active phases through a compartmentalizing strategy toward nanostructured catalyst supports

The design and synthesis of robust sintering-resistant nanocatalysts for high-temperature oxidation reactions is ubiquitous in many industrial catalytic processes and still a big challenge in implementing nanostructured metal catalyst systems. Herein, we demonstrate a strategy for designing robust nanocatalysts through a sintering-resistant support via compartmentalization. Ultrafine palladium active phases can be highly dispersed and thermally stabilized by nanosheet-assembled γ-Al₂O₃ (NA-Al₂O₃) architectures. The NA-Al₂O₃ architectures with unique flowerlike morphologies not only efficiently suppress the lamellar aggregation and irreversible phase transformation of γ-Al₂O₃ nanosheets at elevated temperatures to avoid the sintering and encapsulation of metal phases, but also exhibit significant structural advantages for heterogeneous reactions, such as fast mass transport and easy access to active sites. This is a facile stabilization strategy that can be further extended to improve the thermal stability of other Al₂O₃-supported nanocatalysts for industrial catalytic applications, in particular for those involving high-temperature reactions.

The design and synthesis of robust sintering-resistant nanocatalysts for high-temperature oxidation reactions remains challenging, even though the strategy of metal-support interactions has been extensively used. Here, the authors demonstrate an alternative strategy for designing robust nanocatalysts through a sintering-resistant support.

Palladium doping of In2O3 towards a general and selective catalytic hydrogenation of amides to amines and alcohols

The first general heterogeneous hydrogenation of amides to amines and alcohols is performed under additive-free conditions and without product de-aromatization by applying a Pd-doped In₂O₃ catalyst.

PdO/PdO2 functionalized ZnO : Pd films for lower operating temperature H2 gas sensing

Noble metals and their oxide nano-clusters are considered to be the most promising candidates for fabricating advanced H₂ gas sensors. Through this work, we propose a novel strategy to grow and modulate the density of PdO/PdO₂ nanoparticles uniformly on nanostructured Pd-doped ZnO (ZnO : Pd) films by a one-step solution approach followed by thermal annealing at 650 °C, and thus to detect ppm-level H₂ gas in a selective manner. The gas sensing properties of such hybridized materials showed that the PdO-functionalized ZnO samples offer significantly improved H₂ gas sensing properties in an operating temperature range of 25-200 °C. The deposition of ZnO : Pd films via a simple synthesis from chemical solutions (SCS) approach with an aqueous bath (at relatively low temperatures, <95 °C) is reported. Furthermore, the functionalization of palladium oxide nanoclusters by a simple but highly effective approach on ZnO : Pd film surfaces was performed and is reported here for the first time. The morphological, structural, vibrational, optical, chemical, and electronic properties were studied in detail and the mixed phases of palladium oxide nanoclusters on the ZnO surface were found. Sensor studies of the ZnO : Pd samples (in the range of 25-350 °C operating temperature) showed good selectivity to H₂ gas, especially in the range of higher temperatures (>150 °C, up to 350 °C); however, the PdO/PdO₂ mixed phases of the nanocluster-modified surface ZnO : Pd films showed a much better selectivity to H₂ gas, even at a lower operating temperature, in the range of 25-150 °C. For such PdO-functionalized ZnO : Pd films, even at room temperature, a gas response of ∼12.7 to 1000 ppm of H₂ gas was obtained, without response to any other reducing gases or tested vapors. The large recovery time of the samples at room temperatures (>500 s) can be drastically reduced by applying higher bias voltages. Furthermore, we propose and discuss the gas sensing mechanism for these structures in detail. Our study demonstrates that surface functionalization with PdO/PdO₂ mixed phase nanoclusters-nanoparticles (NPs) is much more effective than only the Pd doping of nanostructured ZnO films for selective sensing applications. This approach will pave a new way for the controlled functionalization of PdO/PdO₂ nanoclusters on ZnO : Pd surfaces to the exact detection of highly explosive H₂ gas under various atmospheres by using solid state gas sensors.

Interplay of the photon drag and the surface photogalvanic effects in the metal-semiconductor nanocomposite

Photon drag effect (PDE) and surface photogalvanic effect (SPGE) can be observed in centrosymmetric media and manifest themselves in photocurrents, the magnitude and polarity of which depend on wavevector and polarization of the excitation laser beam. PDE photocurrent originates from the transfer of the photon momentum to a free charge carrier, while SPGE photocurrent is due to diffuse scattering of the photoexcited carriers in the subsurface layer. However, despite the different underlying physical mechanisms, these photocurrents have almost indistinguishable dependencies on the polarization and the angle of incidence of the excitation laser beam. In this paper, we observe for the first time a competition between PDE and SPGE in the film containing metal (Ag-Pd) and semiconductor (PdO) nanocrystallites. We show that, depending on the angle of incidence, polarization azimuth and wavelength of the excitation laser beam, the interplay of the PDE and SPGE leads to the generation of either monopolar or bipolar nanosecond current pulses. The experiments performed allow us to visualize the contributions both these effects and obtain light-to-current conversion efficiency in a wide spectral range. Our experimental findings can be employed to control the magnitude and polarity of the light-induced current by polarization of the excitation laser beam.

The relationship between the chemical state of Pd species and the catalytic activity for methane combustion on Pd/CeO2

Three Pd/CeO₂ catalysts are synthesized by reduction-deposition and an impregnation method (IMP) to clarify how the chemical state of Pd influences the catalytic performance for CH₄ combustion.

One-step electrodeposition of Pd–CeO2 on high pore density foams for environmental catalytic processes

Electrodeposited Pd–CeO₂ on high pore density foams shows high activity and stability for environmental processes and outstanding mass transport properties.

Synthesis and physicochemical characterizations of coordination between palladium(ii) metal ions with floroquinolone drugs as medicinal model against cancer cells: novel metallopharmaceuticals

Three new complexes of palladium(ii) floroquinolone drugs (levofloxacin (HLVX), pefloxacin mesylate (HPFX) and lomefloxacin (HLMX)) were prepared in alkaline media at pH = 9.

Femtosecond Circular Photon Drag Effect in the Ag/Pd Nanocomposite

We report on the observation of the helicity-dependent photoresponse of the 20-μm-thick silver-palladium (Ag/Pd) nanocomposite films. In the experiment, 120 fs pulses of Ti:S laser induced in the film an electric current perpendicular to the incidence plane. The photoinduced current is a linear function of the incident beam power, and its sign depends on the beam polarization and angle of incidence. In particular, the current is zero for the p- and s-polarized beams, while its sign is opposite for the right- and left-circularly polarized beams. By comparing experimental results with theoretical analysis, we show that the photoresponse of the Ag/Pd nanocomposite originates from the photon drag effect.

In situ dynamic tracking of heterogeneous nanocatalytic processes by shell-isolated nanoparticle-enhanced Raman spectroscopy

Surface molecular information acquired in situ from a catalytic process can greatly promote the rational design of highly efficient catalysts by revealing structure-activity relationships and reaction mechanisms. Raman spectroscopy can provide this rich structural information, but normal Raman is not sensitive enough to detect trace active species adsorbed on the surface of catalysts. Here we develop a general method for in situ monitoring of heterogeneous catalytic processes through shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) satellite nanocomposites (Au-core silica-shell nanocatalyst-satellite structures), which are stable and have extremely high surface Raman sensitivity. By combining operando SHINERS with density functional theory calculations, we identify the working mechanisms for CO oxidation over PtFe and Pd nanocatalysts, which are typical low- and high-temperature catalysts, respectively. Active species, such as surface oxides, superoxide/peroxide species and Pd–C/Pt–C bonds are directly observed during the reactions. We demonstrate that in situ SHINERS can provide a deep understanding of the fundamental concepts of catalysis.

Rational design of heterogeneous catalysts requires molecular understanding of catalytic processes. Here, the authors attach PtFe and Pd nanocatalysts to Raman signal-enhancing Au-silica nanoparticles, allowing them to spectroscopically observe the active species and bonds involved in CO oxidation in real time.

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

Application of surface-science techniques, such as XPS, SXRD, STM, and IR spectroscopy under catalytic reactions conditions yield new structural and chemical information. Recent experiments focusing on CO oxidation over Pt and Pd model catalysts were reviewed.

Nanostructured PdO Thin Film from Langmuir-Blodgett Precursor for Room-Temperature H2 Gas Sensing

Nanoparticulate thin films of PdO were prepared using the Langmuir-Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)-chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air-water interface was confirmed by the surface pressure-area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV-vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed the presence of prominent Pd(2+) with a small quantity (18%) of reduced PdO (Pd(0)) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the band gap energy of PdO was estimated to be 2 eV, which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20-30 nm. These PdO film when employed as a chemiresistive sensor showed H2 sensitivity in the range of 30-4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.

Effects of surface stability on the morphological transformation of metals and metal oxides as investigated by first-principles calculations

Morphology is a key property of materials. Owing to their precise structure and morphology, crystals and nanocrystals provide excellent model systems for joint experimental and theoretical investigations into surface-related properties. Faceted polyhedral crystals and nanocrystals expose well-defined crystallographic planes depending on the synthesis method, which allow for thoughtful investigations into structure-reactivity relationships under practical conditions. This feature article introduces recent work, based on the combined use of experimental findings and first-principles calculations, to provide deeper knowledge of the electronic, structural, and energetic properties controlling the morphology and the transformation mechanisms of different metals and metal oxides: Ag, anatase TiO2, BaZrO3, and α-Ag2WO4. According to the Wulff theorem, the equilibrium shapes of these systems are obtained from the values of their respective surface energies. These investigations are useful to gain further understanding of how to achieve morphological control of complex three-dimensional crystals by tuning the ratio of the surface energy values of the different facets. This strategy allows the prediction of possible morphologies for a crystal and/or nanocrystal by controlling the relative values of surface energies.

The local structure of PdxCe1−xO2−x−δsolid solutions

In this paper, physical methods in combination with quantum chemistry calculations are used to study the local structure of Pd_xCe_1−xO_2−δsolid solutions.

Characterization of hydrogen responsive nanoporous palladium films synthesized via a spontaneous galvanic displacement reaction

A model is presented regarding the mechanistic properties associated with the interaction of hydrogen with nanoporous palladium (np-Pd) films prepared using a spontaneous galvanic displacement reaction (SGDR), which involves PdCl(2) reduction by atomic Ag. Characterization of these films shows both chemical and morphological factors, which influence the performance characteristics of np-Pd microcantilever (MC) nanomechanical sensing devices. Raman spectroscopy, uniquely complemented with MC response profiles, is used to explore the chemical influence of palladium oxide (PdO). These combined techniques support a reaction mechanism that provides for rapid response to H(2) and recovery in the presence of O(2). Post-SGDR processing via reduction of PdCl(2)(s) in a H(2) environment results in a segregated nanoparticle three-dimensional matrix dispersed in a silver layer. The porous nature of the reduced material is shown by high resolution scanning electron microscopy. Extended grain boundaries, typical of these materials, result in a greater surface area conducive to fast sorption/desorption of hydrogen, encouraged by the presence of PdO. X-ray diffraction and inductively coupled plasma-optical emission spectroscopy are employed to study changes in morphology and chemistry occurring in these nanoporous films under different processing conditions. The unique nature of chemical/morphological effects, as demonstrated by the above characterization methods, provides evidence in support of observed nanomechanical response/recovery profiles offering insight for catalysis, H(2) storage and improved sensing applications.

PALLDIUM SUPPORTED MCM-41 MESOPOROUS MOLECULAR SIEVES: SYNTHESIS, CHARACTERIZATION AND CATALYTIC TEST FOR THE REDUCTION OF NO BY CO

Palladium supported MCM-41 molecular sieves with Si/Pd (molar) ratios of 400, 200, 100 and 50 were synthesized by hydrothermal method. All catalysts characterized by various analytical and spectroscopic techniques, viz, XRD, N2 sorption, TEM, DRUV-VIS, Raman. XRD at low angle and high angle studies suggest that the substitution of palladium and presence of palladium oxide phase in the silicate tetrahedral framework structure of MCM-41. TEM investigations confirm the highly ordered cubic structure with irregular distribution of palladium nanoparticles on surface PdMCM-41 . DRUV-VIS and Raman spectra studies indicate the presence of palladium oxide moieties in all PdMCM-41 catalysts. The effect of Si/Pd molar ratio, particle size and reusability with respect to reactor temperature on the catalytic activity of PdMCM-41 catalyst was examined. Comparative study of all these palladium substituted catalysts shows that PdMCM-41(50) catalyst having higher palladium contents shows higher activity at lower temperatures as compared with other PdMCM-41 ( Si/Pd = 100, 200, 400) catalysts.

Active surfaces for CO oxidation on palladium in the hyperactive state

Hyperactivity was previously observed for CO oxidation over palladium, rhodium, and platinum surfaces under oxygen-rich conditions, characterized by reaction rates 2-3 orders higher than those observed under stoichiometric reaction conditions [Chen et al. Surf. Sci. 2007, 601, 5326]. In the present study, the formation of large amounts of CO(2) and the depletion of CO at the hyperactive state on both Pd(100) and polycrystalline Pd foil were evidenced by the infrared intensities of the gas phase CO(2) and CO, respectively. The active surfaces at the hyperactive state for palladium were characterized using infrared reflection absorption spectroscopy (IRAS, 450-4000 cm(-1)) under the realistic catalytic reaction condition. Palladium oxide on a Pd(100) surface was reduced eventually by CO at 450 K, and also under CO oxidation conditions at 450 K. In situ IRAS combined with isotopic (18)O(2) revealed that the active surfaces for CO oxidation on Pd(100) and Pd foil are not a palladium oxide at the hyperactive state and under oxygen-rich reaction conditions. The results demonstrate that a chemisorbed oxygen-rich surface of Pd is the active surface corresponding to the hyperactivity for CO oxidation on Pd. In the hyperactive region, the CO(2) formation rate is limited by the mass transfer of CO to the surface.

A surface x-ray study of the structure and morphology of the oxidized Pd(001) surface

The oxidation of Pd(100) and the formation of PdO was studied in situ using surface x-ray diffraction. A bulklike, epitaxial PdO film is formed at oxygen partial pressures beyond 1 mbar and sample temperatures exceeding 650 K. The main orientation is PdO(001)/Pd(001), based upon bulk reflections from the PdO film. By comparing with measurements from the Pd crystal truncation rods, we estimate an rms surface roughness of 6 A, in good agreement with previous high pressure scanning tunneling microscopy measurements. Finally, we observed the transformation from the (radical5 x radical5) surface oxide to PdO bulk oxide at 675 K and 50 mbar O(2) pressure.

Kinetic hindrance during the initial oxidation of Pd(100) at ambient pressures

The oxidation of the Pd(100) surface at oxygen pressures in the 10(-6) to 10(3) mbar range and temperatures up to 1000 K has been studied in situ by surface x-ray diffraction (SXRD). The results provide direct structural information on the phases present in the surface region and on the kinetics of the oxide formation. Depending on the (T,p) environmental conditions, we observe either a thin (sqrt[5]xsqrt[5])R27 degrees surface oxide or the growth of a rough, poorly ordered bulk oxide film of PdO predominantly with (001) orientation. By either comparison to the surface phase diagram from first-principles atomistic thermodynamics or by explicit time-resolved measurements we identify a strong kinetic hindrance to the bulk oxide formation even at temperatures as high as 675 K.